Time stamping method using time delta in key certificate

Abstract

A method for time stamping a digital document is disclosed. The document originator creates a time stamp receipt by combining the document and a digital time indication. The time stamp receipt is submitted to a time stamping agent having a trusted clock. The time stamping agent optionally validates the time stamp receipt and then computes the age of the time stamp receipt. If valid, the time stamping agent certifies the time stamp receipt by signing the time stamp receipt with a private signature key. The private signature key is selected from a group of signature keys by the time stamping agent based on the computed age of the time stamp receipt.

We claim:  
     
       1. A method for time stamping a document comprising: 
       receiving a time stamp receipt at an outside agency, said time stamp receipt including identifying data associated with said document and a time indication;  
       computing the age of said time stamp receipt based on said time indication;  
       signing said time stamp receipt at said outside agency using a secret signature key, said signature key selected from a group of signature keys based on the computed age of said time stamp receipt.  
     
     
       2. The time stamping method of  claim 1  further including transmitting said signed time stamp receipt to a designated party. 
     
     
       3. The time stamping method of  claim 1  wherein said identifying data comprises a digital representation of at least a portion of said document. 
     
     
       4. The time stamping method of  claim 3  wherein said identifying data comprises a digital sequence derived by application of a deterministic function to at least a portion of said document. 
     
     
       5. The time stamping method of  claim 4  wherein said digital sequence is a hash value derived by application of a one-way hashing function to at least a portion of said document. 
     
     
       6. The time stamping method of  claim 1  wherein said time stamp receipt further includes an identification number associated with the document originator. 
     
     
       7. The time stamping method of  claim 6  wherein said time stamp receipt further includes a sequential record number. 
     
     
       8. The time stamping method of  claim 7  further including the step of validating said time stamp receipt. 
     
     
       9. The time stamping method of  claim 8  wherein the step of validating said time stamp receipt includes comparing said identification number and sequential record number with data maintained by the outside agency. 
     
     
       10. A method for time stamping a document comprising: 
       creating a time stamp receipt including identifying data associated with said document and a time indication;  
       transmitting said time stamp receipt to an outside-agency for certification;  
       computing the age of said time stamp receipt at said outside agency;  
       signing said time stamp receipt at said outside agency using a secret signature key, said secret signature key selected from a group of signature keys based on the computed age of said time stamp receipt; and  
       transmitting said signed time stamp receipt from said outside agency to a designated party.  
     
     
       11. The time stamping method of  claim 10  wherein said identifying data comprises a digital representation of at least a portion of said document. 
     
     
       12. The time stamping method of  claim 11  wherein said identifying data comprises a digital sequence derived by application of a deterministic function to at least a portion of said document. 
     
     
       13. The time stamping method of  claim 12  wherein said digital sequence is a hash value derived by application of a one-way hashing function to at least a portion of said document. 
     
     
       14. The time stamping method of  claim 10  wherein said time stamp receipt further includes an identification number associated with the document originator. 
     
     
       15. The time stamping method of  claim 14  wherein said time stamp receipt further includes a sequential record number. 
     
     
       16. The time stamping method of  claim 15  further including the step of validating said time stamp receipt. 
     
     
       17. The time stamping method of  claim 16  wherein the step of validating said time stamp receipt includes comparing said identification number and sequential record number with data maintained by the outside agency.